Low Poly Style Fire Tutorial/Tips
The easiest, low-poly fire I can think of is the same setup done for trees and plants in many video games: Setup a plane textured with a fire image PNG (with transparency) and create duplicates that you rotate on the Z-axis.

This tutorial can serve as a starting point: https://www.youtube.com/watch?v=6dsP7N3l7kY

A different style with the same principle: https://www.youtube.com/watch?v=hkc-0YUz-Y4

Here is another quick an easy one that can be combined with ones above: https://www.youtube.com/watch?v=OYxb8F5moUo

This last one is about making solid-looking low poly trees, rocks and grass: https://www.youtube.com/watch?v=rtO9maU709k

Finally, it doesn't hurt to lean about particles, they can be simple or complex, realistic or stylized and can even enhance the low-poly ones with sparks. https://www.youtube.com/watch?v=jLjwN_JqUGU